This invention relates to valve gated injection molding, and more particularly to a hydraulically actuated single cavity system with a single inlet in alignment with the valve pin and the gate which provides improved melt flow around the valve pin.
Single cavity injection molding systems with a central inlet are well known in the art. For instance, Gellert U.S. Pat. Nos. 4,026,518 entitled "Bushing Seal for Valve-Gated Injection Mold" which issued May 31, 1977, 4,222,733 entitled "Injection Molding Flow Control Mechanism" which issued Sept. 16, 1980, and 4,286,941 entitled "Injection Molding Nozzle Seal" which issued Sept. 1, 1981 all disclose mechanically actuated systems. More recently, the applicant's U.S. Pat. No. 4,380,426 entitled "Injection Molding Valve Pin Direct Pneumatic Actuator" which issued Apr. 19, 1983 discloses a pneumatically actuated system. Of course, as shown in all of the previous systems, it is necessary that the melt flow passage be offset around at least part of the actuating mechanism which engages the driven end of the valve pin. The melt passage then extends diagonally to join the central bore through the nozzle and the melt flows along the valve pin to the gate. As described in U.S. Pat. No. 4,026,518 mentioned above, a valve bushing is often used to facilitate the manufacture of structure wherein the melt passage joins the valve pin bore.
This previous arrangement wherein the melt passage joins the valve pin bore at an angle is suitable for many applications. However, when molding some materials such as white polyvinyl chloride the pattern of flow of the melt from the melt passage around the valve pin creates discoloured streaks where the flow joins again on the other side. This also delays the elimination of the previous coloured melt when making colour changes because there is slower melt flow on the side of the valve pin opposite the melt passage inlet.